This invention relates to the field of color photoengraving, and more particularly to the exposure of color correction masks and color separation negatives used for color printing.
Printing plates used in photoengraving can only print one color of ink at a time on a piece of paper stock. In order to print a color photograph from a color transparency such as a color slide, the paper stock must be printed in succession with three or more different plates, each having a different colored ink. In four-color photoengraving, which is in wide use, the inks are usually yellow, cyan, magenta and black.
In order to manufacture the printing plates, the components of the primary colors in the transparency are separated and recorded on separate pieces of film known as color separation negatives. The separation negatives are made by exposing specially adapted film to properly filtered light passed through the transparency. The light may also be passed through a properly registered color correction mask, which will be described.
In four color photoengraving, red, green and blue filters are used to separate the components of the primary colors and record them on three separation negatives. Another filter is used to add depth and shape to the photographed image by recording particularly dark portions of the picture on a fourth negative.
The plates are engraved so that when the paper stock is printed with all of the plates properly registered, the inks combine to recreate the original color scheme as accurately as possible, or as accurately as desired. In four-color photoengraving, each primary color is printed in its corresponding substractive color. Thus, cyan ink prints the red-filter record, magenta ink prints the green-filter record, yellow ink prints the blue-filter record, and black ink prints the dark areas of the picture.
Commercially available inks do not print colors precisely. For example, cyan ink behaves as if it were contaminated with some yellow and magenta ink, and absorbs some blue and green light. For this reason, color correction masks are made which compensate for undesired properties of the inks.
The color correction masks are made in much the same manner as the separation negatives, by exposing a piece of specially adapted film to light through a color transparency and an appropriate color filter. One such film is Kodak Pan Masking Film.
The exposure of the masks is difficult and time-consuming because each mask is mounted for exposure one at a time, and they must be exposed in an otherwise totally dark room, without the benefit of dark room lamps. As a result, lights must be turned on and off constantly throughout the process to change the filters as the masks are exposed and to adjust the equipment for the various exposures, or all of these activities must be performed by the operator in total darkness.
Each of the exposed masks is developed by bathing it in proper chemicals for predetermined time periods. Data published by Kodak indicate that under certain circumstances the development time for Kodak Pan Masking Film, for example, may be the same for each mask. However, Kodak recommends that the development times for the masks be adjusted to achieve desired contrasts, and in practice the time that each mask is developed varies, so separate pieces of film are used for the various masks so that they may be developed individually. Thus, in the four-color process, four pieces of film are needed to make the masks, if four masks are used.
The mask developing process is also difficult and time-consuming because the four masks must be developed one at a time and the chemical compositions and temperatures of the chemical baths must be carefully controlled during the entire process. The development process is further complicated by the common practice of obtaining desired contrast indices by adjusting the development times of the masks.
A popular film for color separation negatives made from transparencies is Kodak Separation Negative Film Type I. Data published by Kodak indicate that the development times required to achieve certain contrast indices using that film is different for the four negatives. For this reason, the separation negatives are generally developed one at a time.
Exposure of four negatives for development one at a time is time-consuming and difficult because four exposure set-ups and adjustments must be made, and the exposures must be made in total darkness. Development is also difficult and time-consuming because the development process must be performed four times, and chemical temperatures, chemical compositions and the like must be maintained within strict parameters during the entire time.
In order to develop the four negatives for different periods of time, they must be exposed on separate pieces of film. So in addition to the four pieces of film required for the correction masks, if there are four masks, four more pieces of film are needed for the separation negatives.
The known methods of making color correction masks and color separation negatives just described require the use of as many as eight pieces of film, because each mask and each negative must be exposed and developed individually. This is inefficient and relatively expensive, in part because each piece of film must have enough excess film around its edges to allow for registration holes and adequate borders. Thus, there is a need for methods and apparatus for making color correction masks and separation negatives which use film more efficiently and less expensively.
Exposing and developing masks and negatives one at a time is difficult and time-consuming, as has been described. Thus, there is also a need for processes and apparatus for making color correction masks and color separation negatives which are less time-consuming and less difficult than known processes and apparatus.
Accordingly, an object of this invention is to provide new and improved methods and apparatus for exposing a plurality of color correction masks on a single piece of film so that film is used more efficiently.
Another object of this invention is to provide new and improved methods and apparatus for exposing a plurality of color correction masks on a single piece of film so that the masks may be developed simultaneously under the same conditions.
Still another object is to provide new and improved methods and apparatus for exposing a plurality of color separation negatives on a single piece of film so that film is used more efficiently.
Yet another object is to provide new and improved methods and apparatus for exposing a plurality of color separation negatives on a single piece of film so that the separation negatives may be developed simultaneously under the same conditions.
A still further object is to provide new and improved methods and apparatus for presetting the exposure time for a plurality of color correction masks and exposing the masks automatically, without further adjustments or interruptions.
Yet another object is to provide new and improved methods and apparatus for presetting the exposure time for a plurality of color separation negatives and exposing the negatives automatically, without further adjustments or interruptions.